Lampreys, one of the only two surviving groups of agnathan (jawless) vertebrates,contain several anadromous species that, during their life cycle, thusmigrate from fresh to seawater and back to freshwater. Lampreyshave independently evolved the same overall osmoregulatory mechanismsas the gnathostomatous (jawed) and distantly related teleostfishes. Lamprey gills thus likewise play a central role in takingup and secreting monovalent ions. However, the ultrastructuralcharacteristics and distribution of their epithelial cell types[ammocoete mitochondria-rich (MR) cell, intercalated MR cell,chloride cell and pavement cell] differ in several respectsfrom those of teleosts. The ultrastructural characteristicsof these cells are distinctive and closely resemble those ofcertain ion-transporting epithelia in other vertebrates, forwhich the function has been determined. The data on each celltype, together with the stage in the life cycle at which itis found, i.e. whether in fresh or seawater, enable the followingproposals to be made regarding the ways in which lampreys usetheir gill epithelial cells for osmoregulating in hypo- andhypertonic environments. In freshwater, the intercalated MRcell takes up Cl– and secretes H+, thereby facilitatingthe uptake of Na+ through pavement cells. In seawater, the chloridecell uses a secondarily active transcellular transport of Cl–to provide the driving force for the passive movement of Na+through leaky paracellular pathways between these cells.